Currently it is not possible to focus RadButton through tabbing navigation unless the TabIndex property is set. For the time being TabIndex of 0 can be set, so that the tab order on the page layout is not distracted. For example:

    <script type="text/javascript">
        function pageLoad() {
            //Getting the button's wrapper
            button1 = $find("<%=RadButton1.ClientID %>");
            buttonWrapper = $telerik.$(button1.get_element());

            //Set tabindex attribute to the span element
            buttonWrapper.attr("tabindex", "0");      
        }                 
    </script>

Note that TabIndex=0 is set to the span wrapper element of the RadButton's input with JavaScript as setting RadButton's TabIndex property to 0 will not take effect.